1. Field of the Invention
This invention relates in general to devices for sealing the annulus between two cylindrical members, and in particular to packoffs for sealing off an isolated zone of well tubing.
2. Description of the Prior Art
Once a well has been drilled from the earth's surface to an oil bearing formation, oil either flows to the surface naturally or it does not. About nine out of every ten oil wells in the United States require artificial lift to get the oil to the surface. Various pumping systems have been applied to this task, including electrical and hydraulic downhole pumps. However, one of the most popular means of artificial lift is not a pumping system, but a system known as gas lift.
In the gas lift system, gas is injected into the fluid column downhole to cause oil to flow or to flow more abundantly. A series of gas lift valves are placed at calculated intervals in the tubing string. Gas is then pumped down into the well through the annulus between the tubing string and the well casing. When a gas lift valve is opened, gas enters the tubing string from the annulus. The gas, being much lighter than the oil, reduces the weight of the oil column, causing the oil to flow to the surface.
One means of mounting the gas lift valves in the tubing string is by the use of packoff installations. A packoff installation has a gas lift valve mounted between a pair of packoffs. Tubing or collar stops are set at specific locations and holes are perforated in the tubing string. The packoff installation is run down into the tubing string, contacting a stop so that the packoffs are on either side of the hole in the tubing string. When the packoffs are set the casing pressure cannot enter the tubing until the gas lift valve is opened. An example of this type of packoff installation is shown in FIG. 4 of U.S. Pat. No. 3,278,192 (Tamplen), issued Oct. 11, 1966. Packoffs often experience high differential pressures between the casing pressure in the annulus and the tubing pressure in the tubing string, either the casing pressure or the tubing pressure being greater.